The multicellular spheroid tumor model system developed by Sutherland and co-workers has proven to be a useful test system in radiobiology. Above 400 microns in diameter, spheroids consist of an outer layer of actively metabolizing asynchronous cells which surround a contact inhibited G1 (G0) population. At a depth of 100-150 microns a subpopulation of these G1 cells appears hypoxic radiobiologically and these surround a central necrotic region. We first propose to employ oxygen and pH ultramicroelectrodes with tip diameters of 1-5 microns to measure the levels of pO2 and pH within these spheroids and to study how these parameters vary as the external environment (e.g. pH, pO2, temperature) is altered. A clear definition of these parameters is necessary to facilitate the interpretation of survival experiments employing spheroids. Using the well-defined system, we then propose the effects of combined radiation and chemotherapy in spheroids in an attempt to sensitize radiation resistant hypoxic cells. The drugs to be employed include both electron-affinic, "oxygen mimic" sensitizers such as 5' nitrofuran and presumed indirect radiosensitizers which act to inhibit proliferating cell consumption of oxygen, thereby allowing for greater oxygen diffusion distances and reoxygenation. Drugs of this latter type include, but are not limited to, chlorambucil, dimethyl myleran and BCNU. In this system we will be able to determine independent levels of cell inactivation by both radiation and drugs as well as any enhancement in cell killing when the two modes are combined. Employing the electrodes in the multicell system we will also determine if the observed drug enhancement of radiation cell killing is due to reoxygenation or drug "oxygen mimic" effects. We hope that this proposal will lead to a rapid and effective screening system for hypoxic cell radiosensitizers.